But that's not an argument against all middlemen. Do you think your fishmonger has any value?
Not against all middlemen. But it's a response to your argument:
> People would just cut them out if all they did was skim a bit from everyone.
The point is that "just cut them out" is not so simple in many cases. See TicketMaster and the fact that even artists hate them but have to deal with them.
If you don't know how that works, just look e.g. at TicketMaster:
https://en.wikipedia.org/wiki/Ticketmaster#Criticism_and_con...